Downhill soil movement, such as that which occurs during landslides and ground creep, can cause substantial damage to various structures such as buildings, highways and railroads. Such damage can have severe economic consequences.
According to one contemporary attempt to mitigate such damage, landslides are excavated, subsurface drains are installed, and the land is then backfilled to form a stable slope. This method is frequently successful with respect to mitigating further undesirable movement of the soil.
However, where structures are involved or where space limitations prevent the use of excavation equipment, then in-place stabilization is generally required. To date, the practice of in-place slope stabilization has generally been limited to the use of retaining walls, buttresses, pilings, and soil nailing techniques.
Conventional retaining walls provide a resisting force by virtue of the stiffness of the wall and resistance of its foundation to overturning and base sliding. The stem of a retaining wall is cantilevered from the foundation and is at a substantial mechanical disadvantage with respect to undesirable potential movement. Thus, such retaining walls typically require an extensive foundation in order to adequately resist failure.
Excavation of soil for an extensive foundation at the base (or toe) of a landslide can remove the precarious support presently available and thus inadvertently mobilize the slide. Tie-back anchors used with retaining walls can reduce, or eliminated, the need for such an extensive foundation. However, expense and space limitations sometimes preclude this option.
Mechanically stabilized earth (MSE) retaining walls rely on the tensile properties and pullout resistance of embedded geogrids, as well as their connection strength to an earth retaining facing. For a MSE wall to provide additional resistance to landslide forces, the wall backfill (with its layers of geogrid tying it together) must be massive enough to act as a gravity wall, thus requiring a large abutting fill. Similarly, a buttress is usually constructed of free-draining rock, and can act as a gravity wall provided that it is massive enough. Both MSE walls and buttresses require significant space and a stable foundation at the toe of the landslide. However, the necessary space and foundation are sometimes not available.
Piling and drill piers can be sized to have the mechanical strength required to resist most downhill movements and do not consume a large area. However, piling and drill piers are relatively costly and require large and expensive installation equipment. Piling and piers can be use in conjunction with tie-back anchors to reduce their structural size. However, expense sometimes preclude this option.
Soil nailing utilizes a relatively large number of rods driven though an unstable soil mass and into a more stable underlying layer, as disclosed in U.S. Pat. No. 6,524,027. Soil nailing provides a holding force by virtue of the soil shear resistance around the individual rods. Soil nailing requires a considerable understanding of soil mechanics and the engineering uncertainty of the magnitudes of the shear resistance around the rods makes design difficult. Although empirical designs have been somewhat successful, such designs are generally undesirable because they do not typically have a known factor of safety.
A unique method of in-situ stabilization using a plasma arc torch to vitrify the soil across the sliding plane is disclosed in U.S. Pat. No. 5,181,797. This method requires that multiple borings be made, through which zones or columns of vitrified soil are formed to provide resistance to movement. The resistance to movement is provided by the soil shear resistance around the individual hardened areas, thereby increasing the overall shear strength of the soil mass in a fashion similar to soil nailing. Consequently, this method also suffers from the engineering uncertainties associated with quantifying the magnitude of the resulting increase in shear strength.
As such, although the prior art has recognized, to a limited extent, the problem of undesirable soil movement, the proposed solutions have, to date, been ineffective in providing a satisfactory remedy wherein space limitations exists. Therefore, it is desirable to provide a soil stabilization system that requires relatively small and inexpensive equipment, that requires no additional space (or land area), and that is relatively inexpensive to perform when compared to the other soil stabilization methods.